Fluid catalytic cracking (FCC) is employed in petroleum refineries to convert high boiling hydrocarbon fractions of crude oil to more valuable products like Liquefied Petroleum Gas (LPG), gasoline and diesel. For this, heavy crude oil is chemically cracked into lighter hydrocarbon fractions having comparatively smaller chain of carbon atoms with the help of one or more catalysts. These high boiling hydrocarbons fractions are then introduced, in multiple streams, into a riser reactor section to undergo catalytic cracking. This results in lighter hydrocarbon fractions, which may be further sent to a fractional distillation column for extracting aforementioned valuable products.
In Fluid Catalytic Cracking (FCC), the atomization of hydrocarbon feed is very critical for contacting the hydrocarbon feed with catalyst particles. A uniform and narrow distribution of droplet size helps in faster vaporization of hydrocarbon feed leading to reduction in coke and better product selectivity.
In FCC, catalyst particles having particle size distribution in the range of 0-150 μm with average particle size of 70-90 μm are used to carry out the cracking reaction of hydrocarbon feed. Naphtha, which is a light hydrocarbon feed, normally has a boiling point upto 180 degree celsius. Heavy hydrocarbons such as vacuum residue normally boils over 370 degrees Celsius.
The hydrocarbon feed is injected into the moving catalyst particles (said catalyst particles having temperature greater than 650 degree Celsius) from an apparatus for cracking in the form of droplets and the cracking of these feed molecules takes place in vapour phase on the active catalyst surface in a very short period of time. If the feed is injected without proper atomization, the contact of the feed droplets and catalyst particles will be poor and the heat transfer from the hot catalyst particle to feed will be less, resulting in low vaporisation of feed. Therefore, the hydrocarbon feed is required to be atomized into fine droplets which are of similar sizes of catalyst particles. This essentially helps to increase the contact of feed with the catalyst particles and the transfer of heat from the catalyst to feed for faster vaporization.
Uniform feed atomization will favour catalytic cracking, resulting in more desirable products and decrease in production of undesirable product (coke and dry gas). While designing an apparatus for catalytic cracking, the objective is to generate a narrow distribution of droplet size of hydrocarbon feed with sauter mean diameter (SMD) nearly equal to the average particle size of the catalyst particles. Bigger droplets will cause more penetration into the catalyst bed in riser and form coke and dry gas. Smaller droplet size will cause less penetration.
US20120302805 discloses a feed nozzle assembly comprising at least one primary mixing chamber, a secondary mixing chamber and a tertiary mixing chamber and also a contraction and expansion member. A primary mixture is formed in the primary mixing chamber on introducing a diluent and a hydrocarbon feed. The primary mixture is thereafter introduced into the secondary mixing chamber wherein a first portion of dispersion stream is introduced into said mixture to form a secondary mixture. A second portion of the diluent/dispersion stream is further introduced into the secondary mixture in a tertiary chamber to form a tertiary mixture. The contraction member in said apparatus is used to increase the kinetic energy of the secondary mixture entering into the tertiary chamber.
U.S. Pat. No. 6,902,707 describes a FCC feed injector wherein the atomizing medium is injected at multiple stages to decrease the feed droplet size. The feed injector comprises a plurality of inlets and plurality of mixing zones. The mixing zones are in fluid connection with each other. In one embodiment, the injector comprises an external sparger configured to define a first mixing zone. In another embodiment, the injector comprises a mixing tee configured to define the first mixing zone. The first mixing zone receives the first atomizing fluid and the hydrocarbon feed to form a first mixture. The second mixing zone receives a second atomizing fluid and the mixture from the first mixing zone to form a second mixture. The second mixture is, thereafter, dispensed into the riser reactor zone in a pre-determined spray pattern.
U.S. Pat. No. 6,669,104 B2 discloses a liquid atomization apparatus having an inlet and an outlet. An impingement mixing zone and a shear mixing zone are located between the inlet and the outlet. The fluid introduced into the inlet is initially divided into two separate streams which are then passed into and through an impingement mixing zone in which they are mixed to form a single stream. The mixed, single stream is then passed into and through a shear mixing zone and then into a lower pressure expansion zone, in which atomization occurs to form a spray of atomized drops of the liquid.
CN202290379U discloses an apparatus comprising a primary steam pipe arranged in an outer sleeve. The said primary steam pipe comprises a primary steam spray hole I at the end socket and a primary steam spray hole II at the front end of the primary steam pipe. The apparatus further comprises an inner sleeve and a raw oil inlet pipe tangential with the outer sleeve. The apparatus further comprises a contraction segment, a throat segment and an expansion segment. As secondary steam chamber is formed between the inner sleeve and the outer sleeve. A secondary steam spray hole is arranged at the end socket of the secondary steam chamber and a nozzle orifice is also provided.
CN 201372270 discloses heavy oil feed nozzle comprising an outer sleeve and an inner sleeve concentric to each other. The said nozzle comprises injection of hydrocarbon feed and the steam at multiple stages. The said nozzle comprises a contraction and expansion member. The said nozzle is characterized by multiple numbers of inlets for injection of hydrocarbon steam and diluent/dispersion stream at various stages.
1987/MUM/2015 discloses an apparatus for mixing and atomizing hydrocarbon stream with diluent/dispersion stream. The said apparatus forms a thin film of hydrocarbon and cross shears the same with dispensing diluent/dispersion stream. The said apparatus is used for mixing and atomization of light hydrocarbon feeds. The said apparatus does not comprise expansion and contraction zones.
As can be seen from the prior arts, there has been constant improvements in the apparatuses and methods for mixing and atomizing a hydrocarbon stream using a diluent/dispersion stream. However, there is a further room for improvement in such apparatuses for achieving a desired distribution of droplet size, especially in case of heavy hydrocarbon feeds. It is also required that the construction of such apparatuses is not complicated and is simple and reliable.